poly-phase induction motor 1

[nandarora]

Can anyone tell me that what will happen if I make the rotor of induction generator to rotate at more than double the synchronous speed i.e. I mean that how will the machine behave if the slip is less than -1.Will it simply behave like an asynchronous generator or will there be some breaking in generator?
I even can't even find the torque slip curve for slip less than -1. Most of the sites do not mention about slip being less than -1.
For reference a link is given below from where I was studying Poly-phase Induction motor:

http://electricalstudy.net/lesson/torque-slip-character-sticks/

http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/

 

[flexoprinting]

Well think about the definition of "Slip", what your describing is not slip.

Chuck

 

[waross]

That depends a lot on how you intend to "make the rotor of induction generator to rotate at more than double the synchronous speed".
I have seen several motors running over speed, and the characteristics are different in each case.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nandarora]

when the rotor will rotate at more than double the synchronous speed in direction of the rotating magnetic field then slip will be less than -1. Can anyone refer me a link where i can study such character-stick curve for induction machine.

 

[flexoprinting]

There are many links I found with great info, I don't see anything that specifically addresses your question though. Bill your the "Big Kahuna" when it comes to Gen sets, I'm thinking the output will be close to zero. The way I understand it is the full rated power of induction generator is reached at around 3% of syncronus speed.
As I understand it counter torque reaches a max value and if you rotate past the stability point you see Breakdown torque where the operating conditions get unstable.

The rotating magnetic field torque works to counter the overspeed of rotor because of induced motion in the opposite direction.

So instead of doubling output as one might think with twice the speed you would get no output without an induced magnetic field. Am I understanding it right?

Chuck

 

[nandarora]

yes you are right.
with more than double the synchronous speed in direction of rotating magnetic field the machine will act like a generator. All I wanted to know was that will there be any kind of action equivalent to breaking action that usually happens when slip is between 1 to 2.
because this pat of the character-stick curve is mirror image of the part with slip between 1 to 2.

 

[nandarora]

for reference see the character-stick curve on the 4th figure of the page displayed by the following link:

http://electricalstudy.net/lesson/torque-slip-character-sticks/

Why the portion of curve with speed above twice the synchronous speed been shown in generating action?
Will there be something similar to braking that will take place here as this part of the curve is mirror image of that part.

 

[waross]

I am going to ignore factors such as windage centrifugal force and bearing life. I will also leave to others the discussion as to how to excite the motor for maximum back EMF. I will discuss the basic electrical issues.

A motor generates a back EMF when it is running normally. The back EMF is close to the rated voltage. This is what limits the current through the motor. The winding impedance is quite low, but it is not the line voltage driving the current through the windings that determines the current, it is the difference between the line voltage and the back EMF that drives the current through the windings.

A fully loaded motor may turn at 1750 RPM.
The slip is 1800RPM - 1750RPM = 50 RPM or 50RPM/1800RPM = 2.7777%
So if the motor is driven at 1800 RPM as an induction generator, loaded up to full rated current, it will be generating at 60 Hz x (1 - 0.02777) = 58.333 Hz.
The voltage applied to a motor is limited by magnetic saturation, but the maximum allowable voltage is frequency dependent.
We speak of the Volts per Hz ratio of a motor. (V/Hz ratio)
For a motor rated at 460 Volts at 60 Hz the v/Hz ratio is 480/60 = 8
If this motor is used on a 50 Hz circuit the maximum allowable voltage is 50 Hz x 8 = 400 Volts.
If 480 Volt 60 Hz motor is driven at a little over twice synchronous speed, the frequency will be close to 120 Hz and the back EMF
Will be close to 960 Volts.
Induction generators may work as stand alone machines with capacitors to aid in the exciting.
Simple induction generators work best when connected to assist a grid or synchronous generator and are driven a few Hz above synchronous frequency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nandarora]

Are you sure that you are right?
Here I am talking about generating action and in generating action. In generating mode the frequency of the rotor will be slip*supply frequency.
Even the frequency at the stator supply remains same as is the bus voltage.
So, I don't understand how you are changing the stator end frequency with change in rotor speed.
and even the formula you used f(1-s) is not for the generating mode but of the degenerative breaking mode or rheostatic breaking mode.

In case you have the given example from a book or link then please mention that link so that i can understand properly what you exactly want to say.

just look at the third note in the following link:

http://electricalstudy.net/lesson/reaction-mmf-in-stator/

also have a look at the note in the following link:

http://electricalstudy.net/lesson/poly-phase-induction-generator-or-asynchronous-generator-2/

And unfortunately my question still remained unanswered that will there be something similar to braking action in slip between -1 to -2
as this portion is mirror image of the part of the curve with slip 1 to 2 which is the braking mode for motoring.

And i also want to know that will there be any limit for negative slip (theoretically) as we can move the rotor with primeover with even 4 to 5 times the synchronous speed taking the slip value to -3 or even -4.

 

[Skogsgurra]

To reduce (cannot hope to eliminate) further speculations:
1. At slip 2 (or -2) the rotor frequency is so high that the rotor impedance is almost purely inductive. It is also very high.
2. That means that torque is very low. If you plot the torque curve you will see that the torque (for NEMA rotor designs A and B) is quite low and decreases when you move to slips >1. The reason that it isn't low in D designs is that the rotor bars are designed to have a high resistance at high frequencies (current is forced from thick section of the bar to tinner section).
3. The torque curve is symmetrical around s=0, so you can mirror the curve to get torque at s<0, i.e. oversynchronuous speed.
4. You will then see that torque is quite low also at high negative s.

Better still, have a look at the Ossanna Diagram. It says it all. Albeit in a way that needs some effort to interpret. But once you have grasped it, you will have a very good understanding of the induction motor. A closer study of the torque equation also helps, but don't trust the simplified ones (Kloss etc) too much at extreme slips.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[nandarora]

So, you mean that it is possible to have -ve slip of about -3 to -4 but the torque will be very low due to low current and high leakage impedance.

 

[Skogsgurra]

Yes, the torque stay almost constant because when frequency and induced EMF in rotor increases, so does the wL impedance. The EMF/wl stays very near constant. wL stands for omega*L.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

You seem to be talking about two different effects.
When an induction generator is grid connected, the grid sets the frequency. If the speed is more than 2% or 3% above the synchronous frequency or if the slip frequency is more than 2% or 3% above or below synchronous speed, the motor will overheat.
referring to the first ink you posted; Look at the bottom figure.

http://electricalstudy.net/lesson/torque-slip-char...

For grid connection refer to the vertical line shown as speed-100, slip-0
Now look at the point marked full load operating point and the point marked n_s.
Normal motoring operation is between these points.
A slip of 1 occurs only during the first moment of starting and under fault conditions. Sustained operation with a slip of 1 will result in motor destruction from excess heat in less than a minute.
Again, normal operation is with very low slip values.
Generating;
In the figure shown, generating is the area below the center base line. If synchronous speed is n _s Then motoring is n_s minus slip and generating is n_s plus slip. Slip in both cases is in the order of a few percent.

When the induction generator is islanded (running independently of the grid) the frequency is determined by the speed and by the slip.
Referring back to the figure posted in the link, when the induction generator is islanded the vertical line on the figure shown as speed 100 or n_s will shift with the driven speed, and the load frequency will be lower within a few percent of the synchronous frequency.
While the figure shows the range of -100 speed to +200 speed, (in percentage) and slip as -2 to +1 (in per unit) {confusing},
and while slip of 1 is important for brief periods during starting, safe sustained operation of either a motor or a generator is in the order of 97% speed to 103% speed or -0.03 to + 0.03 slip.
The "U" frame motors in common use 50 years ago were often "plugged" or reversed or stopped by the application of a reverse rotating magnetic field, basically throwing the motor into reverse while running at full speed forward.
Plugging when the way of the Dodo with the introduction of the "T" frame motors common today, with much shorter allowable locked rotor times.
Referring again to the figure, you will see that the torque in the region of -100 to 0 speed is less than the torque in the region from 0 speed to +100 speed. The time to slow the motor to a stop is greater than the time to accelerate the motor to full speed. As a result the heating of the motor may be several times as great during the deccelleration time as during the normal starting period.
Another effect to be aware of:
When a generator is grid connected, an increase in the driving torque results in the generator picking up more of the load from the other generators on the grid.
When a generator is islanded, an increase in the driving torque results in the generator frequency increasing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Bill, I think that you missed the point. OP says: "I am talking about generating action and in generating action. In generating mode the frequency of the rotor will be slip*supply frequency. Even the frequency at the stator supply remains same as is the bus voltage". He also says: "can't even find the torque slip curve for slip less than -1"

So, the question is not at all about normal operation at a slight overspeed. It is about running the motor at twice the synchonous speed, or higher. You are then in the "locked rotor" part of the speed/torque curve and at 2+ times synch speed you are in the plug braking part of the curve. It is not a very useful region to run in. The Ossanna diagram at

http://www.google.se/url?sa=t&rct=j...cd=1&cad=rja&uact=8&ved=0CCEQFjAA&url=http://

http://www.ew.tu-darmstadt.de/media...=5OtnG2oBkT56n5sW6fN0gg&bvm=bv.90790515,d.bGQ

shows this (look at the final frame, frame 5). It covers all slips from s=1 (starting) to s=-.4 (40% overspeed) and it can be clearly seen that the stator current is: 1) high, 2) almost purely reactive and 3) doesn't change much if you go from s=-.4 to s approaching infinity. The latter is because the ratio EMF/wL is rather constant at high slips and it doesn't matter if that slip is positive or negative.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[flexoprinting]

I'm thinking the O.P. is struggling to understand the theoretical nature of motor behavior considering his study materials given, the O.P. keeps referencing the same source and I believe is looking for another source of study on this subject and would like recommendations. I think the O.P. wants to go step by step and completely understand through his own study's.

 

[waross]

Thanks for the clarification Gunnar. I think we are in agreement. I am not sure if the OP intends to run grid connected or stand-alone (islanded). For grid connected, you are absolutely correct. You may have added that operation at twice supply frequency will result in motor burnout in less than a minute.
I almost added to my post and I will do it here:
Any apparent disagreement between my post and Skogsgurra's post is a misunderstanding.
This may be more to the point;
If, for instance, a motor is driving a long, downhill, conveyor, and the conveyor load starts driving the motor over speed, the motor will apply a resistance or breaking action limited to about equal to starting torque. When the speed reaches synchronous speed plus normal slip speed (about 1840 RPM for a motor rated at 1760 RPM) the current will be about equal to rated full load current and the resisting or braking torque will be about equal to normal full load torque. As the driving force increases and the slip increases the resisting torque will increase to about starting torque and the current will increase to about initial starting current. Burnout may occur in about 20 to 60 seconds at 200% synchronous speed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, burning out. So obvious that I didn't think I needed to mention it.

But it is possible to do the experiment if you reduce voltage to 20 or 10 percent. I've done so and ran the machine in a test bench. you learn lots from such "meaningless" experiments.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

"Yes, burning out. So obvious that I didn't think I needed to mention it."
Obvious to those of us who work with motors, yes,
The first line of the first post was:
"Can anyone tell me that what will happen if I make the rotor of induction generator to rotate at more than double the synchronous speed"
We probably should have stated the obvious back here. As the questions kept coming, I Though in may be well to mention the rapid burnout issue. I am not arguing, just explaining my reasons.
I like your reduced voltage test bench setup.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

I do such things now and then. A three-phase Variac with some punch in it is a fine men's toy. And so is a four-quadrant regenerating DC drive. Everyone should have one

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[flexoprinting]

Bill

You did make it quite clear in your post I thought was very informative. The problem I was having following the O.P. post and links he supplied was the spelling of "Breaking" which could refer to break down torque, and "Braking" which of course means slowing of machine. Spell checkers are great except for situations like these. My take away from reading this was good though and I'm sure the O.P. learned something as well would be nice if he kept the conversation going though I thought was interesting. Thanks for your input Gunnar, I need to brush up on my Trig and look into the Ossanna diagram.

Chuck